Interest in fabricating monolithic RF amplifiers in silicon has been prompted by interest in miniaturization of circuits for use in cellular telephones, wireless modems and other types of equipment, especially in high frequency bands such as the 1-GHz band. However, a major limitation in making use of large spiral inductors in monolithic circuits is the low frequency and low-Q self-resonance caused by distributed capacitance to the conductive silicon substrate. One way to avoid this limitation is by adding a process step to etch out the substrate underneath the inductor, thereby suspending the inductor over a pit in the substrate. This approach is described in U.S. Pat. No. 5,539,241 and J. Y-C. Chang and Asad Abidi, xe2x80x9cLarge Suspended Inductors on Silicon and Their Use in a 2-xcexcm CMOS RF Amplifier,xe2x80x9d IEEE Electron Device Letters, Vol. 14, No. 5, May 1993, page 246-248, both of which are incorporated in their entireties by reference herein. There is a need for achieving the same isolation in a standard semiconductor manufacturing process flow.
The invention provides a method for achieving electrical isolation between a spiral inductor and an underlying silicon substrate, usable in a standard semiconductor manufacturing process flow. The invention further provides monolithic inductor circuits utilizing such method.
In accordance with one aspect of the invention, metal 1 guard rings are placed under the spiral turns of an inductor and driven with voltage buffers connected to the inductor metal at several places. This electronically reduces to an insignificant level the effective capacitance to the substrate seen by the inductor. In another aspect of the invention, output resistance is used at the voltage buffers to produce a high-Q resonator. The invention enables practical inductors formed with standard BiCMOS processes to be used in monolithic silicon RF circuits in the range of approximately 500 MHz-2 GHz.